1. Field of the Invention
Various chemical processes such as processes for the oxidation of waste material, utilize an oxygen-containing gas, such as air, at relatively high pressure. The air may be compressed by a centrifugal compressor including a rotor mounted on a shaft. A housing surrounds the rotor and the adjacent portion of the shaft, and bearings are provided to support the shaft in the housing for rotation. Typically, a liquid lubricant is continuously injected into the bearings, flows through the bearings and is drained from the housing for re-injection. The lubricant flows through the bearings in both axial directions; thus a portion of the lubricant for the bearing closest to the compressor rotor will flow axially toward the rotor. At the same time, a certain amount of the air being compressed inevitably leaks from the area between the rotor and housing toward the adjacent bearing. Since the lubricant is combustible and may be at least partially atomized, and the air or other oxygen-containing gas being handled by the compressor is at an elevated pressure, it is particularly important to prevent intermingling of the lubricant and air so as to avoid possible explosion.
Accordingly, a seal may be provided surrounding the shaft intermediate the rotor and the adjacent bearing and sealing between the shaft and housing. However, in many instances it is desirable to use a seal such as a labyrinth seal which is designed to permit a certain amount of leakage therethrough. In any event, regardless of the type of seal used, it may be impossible to completely eliminate leakage across the seal. Thus, additional precautions are desirable.
2. Description of the Prior Art
Prior U.S. Pat. Nos. 3,937,022, 3,831,381, 3,670,850, 3,452,839 and 3,420,434 all disclose the general concept of injecting a seal gas to separate the lubricant and process fluid of a rotary fluid handling machine such as a turboexpander or compressor. More specifically, such a seal gas is injected into a seal located between the bearings and the rotor of such a machine under a sufficient pressure to prevent intermingling of the process fluid and lubricant. However, since such seal gas then typically becomes intermingled with both the process fluid being handled by the rotor and the lubricant being circulated through the bearings, new problems are introduced. In general, the seal gas must be chosen such that it will be compatible with both of the other fluids in the sense that it will not react with these fluids or their constituents or otherwise interfere with their proper functions. Also, it is usually necessary, at least as to the lubricant, that the seal gas be easily separable therefrom before recycling the lubricant. In many such prior art systems, suitable seal gases are readily available. For example, in many such systems a relatively inexpensive seal gas, obtained from an outside source, may be compatible with both the process fluid and the lubricant. In other systems, such as those disclosed in U.S. Pat. Nos. 3,937,022 and 3,831,381, the rotary device being sealed is a turboexpander which may be handling, for example, hydrocarbons. In such cases, a seal gas may be obtained from the process fluid itself.
However, in systems in which air or another oxygen-containing gas is being compressed, or otherwise processed, the problem of finding a suitable seal gas becomes more difficult. Not only must the seal gas be compatible with the air or process fluid and also with the lubricant, but it must also be incapable of supporting combustion and virtually fool-proof in maintaining complete separation of the air and lubricant. In general, in such systems an inert gas is desirable. However, obtaining a suitable inert gas, such as nitrogen, from an outside source or extracting it from the air being compressed would be unduly costly.